This invention relates to a circuit pack housing and more particularly to a circuit pack housing which can be molded as a unitary structure using straight pull tooling and requiring no post molding machining.
The circuit pack housing has become an indispensable part of electrical and electronic circuits. Heretofore, the circuit pack housing or carrier was cast in metal from a sand casting or was assembled from machined parts. A metal circuit pack housing, while having the attributes of strength and durability, is quite expensive to manufacture as compared to the individual circuit packs that are housed in the housing. As such, more attention has focused on developing a less expensive circuit pack housing fabricated from a plastic material.
Priorly, attempts have been made to mold a plastic circuit pack housing as a unitary structure. However, these attempts have not been successful due to shrinkage of the plastic piece part upon cooling in the mold. The shrinkage of the plastic increases the frictional force between the mold and plastic making it difficult to remove the piece part from the mold. Removal of the plastic piece part from the mold under these conditions generally causes damage to the piece part.
Collapsible molds have been designed to separate the mold from the plastic after sufficient cooling has occurred to facilitate removal of the molded part from the mold. However, the cost of designing and manufacturing a collapsible mold is expensive as compared to the cost of designing and manufacturing a non-collapsible mold.
Molding separate identical piece part halves and then assembling the halves to form a housing is still another method of avoiding the problems associated with molding a circuit pack housing. This technique is essentially disclosed in U.S. Pat. No. 4,232,356, issued to Michael A. Saunders, et al, on Nov. 4, 1980. The Saunders patent discloses a circuit pack housing assembled from identical halves. While this method avoids some of the problems discussed above, it is an expensive alternative due to costs of molding and assembling the unit.
it is desirable to take advantage of straight-pull tooling to mold plastic piece parts, such as a unitary constructed circuit pack carrier. Straight-pull tooling is simple to design and is highly reliable since it has no moving parts, other than the mating of the cavity and core.
Presently, molded piece parts are designed to taper in the direction of removal from the straight-pull tooling to reduce the drag between the molded part and tooling. This principle is referenced in the Plastics Products Design Engineering Handbook by Levy and Dubois, published by Van Nostrand Reinhold Company, 1977, and could be applied to mold a unitary circuit pack carrier. However, a circuit pack carrier configured in this manner would not be suitable for housing circuit packs since the vertical spacing between the front and rear of the carrier would vary due to the required taper. As such, circuit packs lodged in the carrier would fit loosely between the non-parallel circuit pack guides at the front of the carrier and therefore, would be responsive to vibration. Over a period of time, vibration would tend to interrupt the electrical connection between a circuit pack and the corresponding backplane wiring.
A circuit pack carrier must be held within close tolerance for proper alignment of the circuit pack connector and corresponding prespaced backplane terminals. This is especially true for a large circuit pack carrier where, for proper alignment of connector and backplane terminals final tolerances must be held to within 5 mils. However, in the art of molding, commonly a tolerance of one mil per inch can be achieved. An objective of one mil per inch applied to a large circuit pack carrier, fifteen to twenty inches in width, would result in a tolerance of fifteen to twenty mils. As such, some of the circuit packs would not properly mate with the corresponding backplane terminals.